1. Field of the Invention
The present invention relates to a fuel supply control device for an aircraft engine and, more specifically, it relates to a fuel supply control device used for a turbo-charged diesel engine for an aircraft.
2. Description of the Related Art
Diesel engines are widely used in various fields due to their high fuel efficiencies. The use of diesel engines is not limited to surface vehicles such as automobiles. It is also known to use diesel engines in aircraft.
For example, Japanese Unexamined Patent
Publication (Kokai) No. 2001-207897 discloses an example of a diesel aircraft engine. In the '897 publication, the fuel injection amount of the engine is corrected based on the exhaust temperature in order to compensate for the changes in the fuel injection characteristics of the fuel injection system due to the number of operating hours of the engine.
When a diesel engine is used in an aircraft, as the atmospheric pressure changes in accordance with the flight altitude, problems that are not found with diesel engine for surface vehicles occur. Further, although a naturally-aspirated diesel engine is used in the aircraft of the '897 publication, a so-called critical altitude problem, which is unique to a turbo-charged diesel engine, occurs if a turbo-charged diesel engine is used in an aircraft.
A critical altitude is defined as the highest flight altitude that allows the aircraft engine to operate at its rated output power.
The maximum output power that a diesel engine can produce is determined by various factors. For example, the maximum output power of diesel engine, i.e., the maximum fuel injection amount is restricted by a “smoke limit”, which is the maximum amount of fuel that can be supplied to the engine without producing smoke in the engine exhaust gas.
The maximum output power is also restricted by the maximum allowable exhaust gas temperature in order to protect the engine. Further, when the engine is equipped with a turbocharger, the maximum output power of the engine is also restricted by the maximum allowable rotation speed of the turbocharger in order to prevent a “turbocharger overrun”, i.e., an over-speed operation of the turbocharger.
In a high-pressure turbo-charged diesel engine, usually, turbocharger overrun is the dominant factor for restricting the maximum output power of the engine.
As the atmospheric pressure becomes lower as the flight altitude becomes higher, if the engine output power is kept at a constant value, the boost pressure must be kept at a constant value, thereby the pressure ratio at which the turbocharger operates must be increased as the altitude becomes higher. Therefore, in order to produce a rated engine output power, the turbocharger speed must be increased as the flight altitude becomes higher.
Therefore, when the engine is operated at a rated output power, the turbocharger speed reaches the maximum allowable speed when the flight altitude increases to a certain value. This flight altitude is the critical altitude. In other words, the critical altitude is the highest flight altitude that allows the engine to be operated at the rated output power without causing turbocharger overrun.
When the flight altitude exceeds the critical altitude, therefore, the fuel injection amount must be restricted in order to keep the turbocharger speed at a speed lower than the maximum allowable speed. Thus, the maximum engine output power becomes lower than the rated output power when the altitude becomes higher than the critical altitude in a turbo-charged diesel aircraft engine.
FIG. 2 shows a general relationship between the maximum output power that the engine is allowed to produce and the flight altitude. As shown in FIG. 2, the maximum output power of the engine agrees with the rated output power of the engine when the flight altitude is lower than or equal to the critical altitude ALcr. However, when the flight altitude becomes higher than the critical altitude ALcr, the maximum engine output power decreases from the rated engine output power as the altitude increases. In an actual turbo-charged diesel aircraft engine, as the maximum output power of the engine corresponds to the maximum limit of the fuel injection amount, the fuel injection amount in the actual turbo-charged diesel aircraft engine is set by the line in FIG. 2 in accordance with the flight altitude.
By setting the maximum limit for the fuel injection amount of the engine by the line in FIG. 2, the engine can produce the rated output power only when the flight altitude is lower than the critical altitude.
However, in the actual operation of an aircraft, the atmospheric pressure changes depending on air temperature as well as on the altitude. Namely, when the air temperature (the ambient temperature) is higher, the density of air decreases and a higher turbocharger speed is required to achieve the same pressure ratio even if the altitude is the same. Further, as the intake air temperature of the engine also becomes higher when the air temperature is higher, the boost pressure must be increased in order to prevent a decrease in the intake air flow rate (mass flow rate) even if the engine output power is the same. Therefore, an increase in the air temperature considerably increases the turbocharger speed. Consequently, the critical altitude becomes lower when the air temperature becomes higher, in the actual operation of an aircraft.
FIG. 3 shows the change in the air temperature in accordance with the altitude.
The line H in FIG. 3 shows the highest air temperature used for the aircraft design (a so-called “hot day conditions” temperature) and the line C shows the lowest temperature used for the aircraft design (a “cold day conditions” temperature). The line S indicates a “standard conditions” temperature that is an average actual air temperature at the respective altitudes.
Since the air temperature varies widely even if the altitude is the same, the critical altitude is usually determined based on the highest air temperature (i.e., the hot day condition) in order to protect the turbocharger even in the worst conditions.
Therefore, when the actual air temperature is lower than that in the hot day conditions, the critical altitude is set at a value lower than the altitude actually required for protecting the turbocharger. This means that, the maximum engine output power is unnecessarily restricted to a value lower than the rated output power in an altitude range above the critical altitude when the condition is other than the hot day condition. The undue restriction in the maximum output power of the engine sometimes deteriorates the operating performance of the aircraft.
This problem may be solved by changing the critical altitude in accordance with the actual temperature conditions. (the air temperature). However, if the critical altitude must be changed in accordance with the temperature conditions, the line representing the maximum limit for the fuel injection amount in FIG. 2 must be changed in accordance with the air temperature. This complicates the fuel injection amount control system of the aircraft engine and, therefore, is not preferable.